1. Field of the Invention
This invention relates to an insulated gate bipolar transistor and a method for manufacturing the same.
2. Background Art
The insulated gate bipolar transistor (hereinafter also referred to as “IGBT”) is a device having a pnpn structure in a semiconductor substrate in which a current is passed along its thickness by injection of electrons from an n-type source layer and injection of holes from a p-type emitter layer.
In an IGBT, to achieve successful tradeoff between the reduction of ON voltage and the reduction of turn-off time, it is effective to restrict the amount of dopant in the entire p-type emitter layer, which is a supply source of holes, to an appropriate range. However, because the p-type emitter layer must form ohmic contact with the positive electrode, its dopant concentration needs to be increased to some extent. To this end, a technique has been developed for forming a thin p-type emitter layer to reduce the total amount of dopant with maintaining high dopant concentration (see, e.g., JP 2002-261282A).
However, this conventional technique has the following problem. In manufacturing the above IGBT having a thin p-type emitter layer, thickness variation of the p-type emitter layer due to process error leads to variation in IGBT characteristics.